Magnetic resonance imaging (MRI) is a major imaging technique used in medicine. MRI is capable of generating detailed images of soft tissues such as the brain, muscles and kidneys. Specific properties of the various compounds found inside tissues, such as water and/or fat, are used to generate images. For example, when subjected to a strong magnetic field, the vector sum of the nuclear magnetic moments of a large number of atoms possessing a nuclear spin angular momentum, such as hydrogen, which is abundant in water and fat, will produce a net magnetic moment in alignment with the externally applied field. The resultant net magnetic moment can furthermore precess with a well-defined frequency that is proportional to the applied magnetic field. After excitation by radio frequency pulses, the net magnetization thus allows a detectable signal to be generated.
The signal generated can be encoded using various mechanisms to allow conversion to spatial images. For example, gradient coils (high power electromagnets) are used to encode spatial information. The spatial encoding is achieved by causing the gradient coils to produce a linearly varying magnetic field with position in an imaging volume within which the image to be scanned is placed. In real world implementations, the field profiles produced by the gradient coils deviate from strictly linear. The amount of deviation depends on spatial position within the image. The deviation from a strictly linear field leads to a spatial warping of the acquired image. Accordingly, there is a need to undo the image warping or “unwarp” the acquired images during processing of the acquired imaging data.